1. FIELD OF THE INVENTION
This invention relates generally to control valves for earth boring tools, and more particularly to a control valve used in the power fluid supply line of impact type earth boring tools to prevent fluid supply to the boring tool until a desired line pressure is obtained.
2. BRIEF DESCRIPTION OF THE PRIOR ART
In certain locations, such as under paved roadways, in highly developed areas or in difficult digging conditions, it may be more efficient and less expensive to bore a small diameter hole horizontally under the ground surface rather than cut a open trench to install gas, telephone, water, electric or other buried utility services. A variety of boring tools are available for this purpose, including rotary flexible rod devices, auger devices, pipe pushers, and air or hydraulic powered impact type piercing tools (also known as percussive moles), the latter being supplied power through flexible hose.
Guided boring systems have been developed to open a relatively long bore hole (several hundred or thousand feet) starting from the power unit driving the boring tool which utilize a small drilling frame or rig, sections of drill pipe or flexible hose, a down hole boring tool or "mole" with a steering assembly, and tracking instrumentation. Typically, hydraulic power is used to control various functions of the drilling frame while compressed air is furnished to the down hole boring tool by means of the drill pipe or hose and swivel devices on the drill frame carriage. The down hole boring tool may be a drilling motor driving a cutting bit to drill through rock or a percussive (impacting) mole for compacting a bore hole in soil.
As these down hole tools progress away from the drilling frame, more drill pipe is added between the boring tool and the drill frame carriage. As each joint of drill pipe is added to the drill string, air flow to the tool must be interrupted and the drill string emptied of air pressure. This is normally accomplished by a shut off or switching valve at the carriage. Once the connection of another joint of drill pipe is completed, the air flow to the down hole tool can be initiated. Thus, the drill pipe is, in effect, forming an ever increasing expansion chamber as the tool advances. A similar problem exists where the drilling mole is operated on the end of a flexible conduit or air hose which is sequentially or incrementally lengthened as the mole progresses into the ground.
Expansion of the compressed air entering the empty drill string drastically reduces the initial air pressure and energy potential available to start the tool in operation. This energy potential builds up slowly because of a limited or fixed capacity for generating compressed air entering the lengthened drill pipe. Since most air compressors have small air tanks, the time required to fill the pipe increases as the length and/or diameter of pipe increases. If pressure build-up inside the mole is slow, the pressure leaks across the hammer and the tool will not start.
The percussive tool in particular requires a certain impulse of energy to initiate operation because of hammer inertia, and internal friction and leakage. This may be further aggravated by ineffective lubrication or frost conditions from air expansion within the tool. In cold atmospheric conditions, a percussion mole may freeze moisture in the tight seal areas. For the down hole motor, similar difficulties could occur from excessive drag on the drill bit. The present invention is installed in the drill string or flexible power supply hose of pneumatic percussive moles which allows an instantaneous, high-pressure blast of air to the downstream percussion tool to overcome the problem of difficult starting conditions such as those caused by long drill strings or hose, a wet borehole or freeze-up conditions.
Several percussion mole steering systems are revealed in the prior art. Coyne et al, U.S. Pat. No. 3,525,405 discloses a steering system which uses a beveled planar anvil that can be continuously rotated or rigidly locked into a given steering orientation through a clutch assembly. Chepurnoi et al, U.S. Pat. No. 3,952,813 discloses an off-axis or eccentric hammer steering system in which the striking position of the hammer is controlled by a transmission and motor assembly. Gagen et al, U.S. Pat. No. 3,794,128 discloses a steering system employing one fixed and one rotatable tail fin. However, these patents do not suggest a control valve associated with the boring tool.
In commonly assigned U.S. Pat. Nos. 4,632,191, and 4,694,913, a steering system is disclosed for percussion boring tools for boring in the earth at an angle or in a generally horizontal direction. The steering mechanism comprises an asymmetric member attached to the anvil of the tool to produce a turning force on the tool and movable tail fins incorporated into the trailing end of the tool which are adapted to be selectively positioned relative to the body of the tool to negate the turning force. Turning force may also be imparted to the tool by an eccentric hammer which delivers an off-axis impact to the tool anvil.
There are several patents which disclose various valves having pressure operated mechanisms, none of which are used in the power fluid supply line of impact type earth boring tools, or utilize the present mechanism to prevent fluid supply to an impact type earth boring tool until a desired line pressure is obtained.
Mason, U.S. Pat. No. 3,180,433 discloses an impacting tool having a latch to prevent actuation of the tool until a predetermined velocity of the drive fluid is reached.
Jacobi, U.S. Pat. No. 2,276,979, Edman, U.S. Pat. No. 2,844,166, and Tennis, U.S. Pat. No. 2,848,014 disclose valves having pressure operated latch mechanisms, but for controlling an air-pressure-operated earth boring tool.
Articles on page 18 in the Autumn 1986 issue of MICROTUNNELlNG magazine (British) and on page 18 of the July 1986 issue of UNDERGROUND magazine (British) mention a percussive hammer having an electrically operated solenoid valve connected to the hammer which is manually operated when to provide the kick or boost required to get the percussive equipment moving properly.
The cited prior art and any other prior art known to applicants does not show an air operated earth boring tool having a control valve which opens automatically to permit flow of air pressure to the tool when the air-line pressure reaches a predetermined level adjacent to the tool and closes automatically at a lower pressure.